In recent years, a great deal of research has gone into techniques for multiplexing optical signals at high density, with the aim of further expanding transmission capacity in optical communication systems. In optical transmission systems that multiplex a plurality of optical signals at high density, deterioration of transmission performance becomes conspicuous due to nonlinear optical effects during long distance transmission. Frequency shift in the order of GHz may occur in the wavelength-tunable light sources commonly used for optical transmission in digital coherent systems, and crosstalk between carriers is a cause of deterioration in transmission performance. In particular, in systems that multiplex subcarriers at high density such as superchannel technology, deterioration of transmission performance due to crosstalk becomes conspicuous.
A conventional invention for compensating for optical frequency shift is described in Patent Literature 1. In the invention described in Patent Literature 1, an optical transmission unit performs transmission after reducing the optical power of a specific wavelength from the wavelengths of a plurality of channels, and an optical reception unit evaluates the channel crosstalk amount from the bit error rate of a channel adjacent to the channel with reduced optical power in order to detect wavelength shift. Then, the optical transmission unit compensates for the detected wavelength shift to control wavelength intervals.